The development of a laser as a convenient source of light that is both temporarily and spatially coherent has given rise to a class of interferometric techniques for non-destructibly testing objects by interferometrically comparing their surfaces in a stressed and unstressed condition. For example, subsurface anomalies in pneumatic rubber vehicle tires may be detected by placing the tires in a pressure chamber and making an interferometric comparison of their surface contours or surface shear strains before and after a pressure change. Some subsurface separations resulting from improper manufacturing procedures in a pneumatic tire or deterioration experienced during its use will result in non-uniform deformation or changes in the shear strain of the tire surface as a result of the pressure change.
Interferometric inspection techniques employing single, double or multiple exposures of coherently illuminated surfaces to a photographic film forming an interferogram may also be employed for non-destructive testing. One technique has been commercialized under the trademark L-RAY.RTM. or the term "SHEAROGRAPHY" as described in Hung et. al U.S. Pat. No. 4,139,302 issued on Feb. 13, 1979. The Hung et al reference discloses a double exposure surface analysis method not requiring a separate reference beam. In the method, light from a coherently illuminated surface passes through a special lens which focuses incoming light from the object to form two or more overlapping and interfering images. A camera stand that can adjustably mount a camera, within the bead diameter of a tire for use in conjunction with the L-RAY.RTM. nondestructive testing method is disclosed in Wright et al U.S. Pat. No. 4,392,745 issued on Jul. 12, 1983.
In the past, it has been found that a typical optical camera lasts approximately 100,000 cycles. The 100,000 cycles provide for a lifetime of use by an average amateur photographer. Most professional photographers use a camera a relatively long time between repairs because they commonly use a plurality of cameras, each with a different lens, to eliminate the time needed to change lenses between shots.
The above-identified interferometric testing methods have been successfully used on a commercial basis. A tire or other tested object is placed in a light tight pressure chamber. A pulse of light emanates from a laser when the shutter is opened to expose the film. The chamber is pressurized and a second pulse of light double exposes the film. The camera or the test object is adjusted to a different position to test a different portion of the tested object. The shutter may be cycled opened and closed for each pulse of light. A camera used in the described interferometric systems often can undergo over one million (1,000,000) cycles a year. The prior art cameras thus would have to be repaired, on the average, 10 times a year. It has been found that most of the needed repairs are due to the inherent nature of the shutter action of a standard camera. The shutter of standard cameras are spring loaded such that upon actuation of the camera, the shutter flies across the aperture and slams to its closed position. The abrupt starts and stops of the shutter shake or vibrate the camera apart over the many cycles it encounters.
What is needed for interferometric analysis is a camera having a shutter that is positively driven between the opening and closing motion and that is buffered when it approaches its open and closed positions.